Single mode photonic circuit architecture and a new optical splitter design based on parallel waveguide mode conversion

ABSTRACT

The new single mode circuit (SMC) architecture is invented for photonic integrated circuits (PIC). This architecture allows using multimode waveguides or structures to construct a single mode operated PIC. The multimode sections used in such SMC based PIC possess strong lateral confinement so that the PIC can have high circuit density and high optical performance at the same time. A parallel mode converter structure is also invented here. Based on this parallel mode converter, a low loss optical splitter can be constructed for high index contrast waveguide system.

PRIOR PROVISIONAL PATENTS

This utility patent application is based on, and claims the benefit ofthe following provisional patent:

Appl. No. 60/810,865.

Filing date: Jun. 5, 2006

Title: Single mode photonic circuits based on multimode waveguide and anew optical splitter design based on parallel waveguide mode conversion

BACKGROUND OF THE INVENTION

In the past, photonic integrated circuits (PIC), which is also calledplanar lightwave circuits (PLC) or optical integrated circuits (OIC),are designed using single mode optical waveguide structures. The reasonsof using single mode waveguide is to ensure that the whole circuits iswithin the single mode operation region, therefore, no high order modeswill be excited both within the circuits and at the interface of thecoupling to the outside world, usually to optical single mode fiber. Ifthe waveguide is not single mode, when the higher order modes areexcited, multiple guiding modes will propagate together along thewaveguides and each mode carries part of signal power. As the result ofthe multimode propagation, the signal will suffer significant modedispersion and random coupling among the modes when discontinuity orstructure change occurs. The coupling from the PIC to the end singlemode fiber will have large and random loss, making the device not usablein the system.

However, the single mode operation condition of the waveguide is, sometime, too restrictive. It will also create problems for the coupling andthe achievable circuit density of PICs. A typical example is thesilicon-on-insulator (SOI) optical waveguide as shown in FIG. 1.

-   <11>: Ridge waveguide cross section;-   <12>: The substrate of the ridge waveguide (bottom cladding);-   <13>: Ridge waveguide core;-   <14>: The ridge;-   <15>: Slab region of the ridge waveguide;-   <16>: Top cladding of the ridge waveguide;-   <17>: The layer structure of the starting SOI wafer;-   <18>: The silicon layer of the starting SOI wafer;

Due to the strong material index contrast between the core <13> and thecladding (substrate) in SOI (n_(f)=3.48 vs. n_(c(f))=1.44˜1.8), forlarge dimension comparable with the regular single mode fiber, thesingle mode condition must be maintained by the weaker lateral effectiveindex contrast, which is between the slab mode effective indices of theregion I and II <15>. The slab region itself, both I and II, aremultimode, but the lateral effective index contrast is so weak that allthe high order vertical mode of slab region I can not propagate withoutradiation into the region II.

Since the vertical index contrast in SOI structure is much stronger thanother material systems, such as silica (SiO₂) and polymer, the SOIwaveguide usually ends up have weaker lateral index contrast than silicaand polymer based waveguide. It makes the SOI waveguide PIC with largecross section has to have larger bending radius, which results in lowcircuit density. To achieve a similar bending radius as silicawaveguide, the waveguide dimension must be reduced to about half of thesingle mode fibers, which causes severe problem in coupling. A 3Don-chip taper usually must be used to reduce the coupling loss.

Another problem with SOI based PIC is the splitter junction loss. Atypical excess loss of a SOI based waveguide splitter due to thesplitter junction is usually ˜1 dB, while the silica based waveguidesplitter has only less than 0.5 dB. The larger junction loss is becausethe high-index contrast between the silicon dioxide (cladding) and thesilicon (waveguide core), and the ridge structure itself. When the modehits the junction, a significant scattering will occur and the fieldgets an abrupt disturbance. An example is demonstrated in FIG. 2, a 1×2splitter junction <21>.

BRIEF SUMMARY OF THE INVENTION

There are two structures are invented here to solve the problem faced bythe SOI PIC, and any other high index material system: the conflictbetween the density of the circuit and its performance. The firstinvented structure, or architecture, or design method, is the singlemode photonic circuit (SMC) in which multimode waveguide can be used.Without the constraint of single mode waveguide condition, the multimodewaveguides used in the SMC have both high lateral index contrast andtherefore the PIC can have small bend radius, resulting in high densityof the photonic circuit, and the high optical performance at the sametime. Also, the thick silicon layer <18> can be used, to make the PICeasy to be coupled with standard single mode fiber.

The second invented structure is a parallel mode converter that can beused to construct a low loss optical splitter in any splitting ratio.With the parallel mode converter, the lightwave splitting is realizedduring the mode conversion between the input single channel waveguidemode and the super modes of the output multiple parallel-coupled channelwaveguide. By eliminating the scattering caused by the oxide between thechannels, the optical splitter can have the excess loss as low as thosebased on regular low index contrast material system.

DETAIL DESCRIPTION OF THE INVENTION

Single Mode Photonic Circuits (SMC) Architecture

The first invented structure, or architecture, or a design approach, isthe single mode photonic circuits (SMC) in which the multimode waveguidecan be used. As we mentioned before, the whole PIC must be maintained assingle mode operation, but it doesn

have to be achieved by the single mode waveguide. The idea is a newconcept: single mode at system (circuit) level. As long as the wholecircuit can only operate at one mode, the individual section can beconstructed by multimode waveguides. A mode filter will be insertedafter the multimode section. If designed correctly, the high order modeswill not be excited in those multimode sections, and even they are, themode filter will remove those high order components and keep the wholesystem at the single mode. In the ideal case, the high order modeexcitation will never occur after the mode filter of the fiber-to-PICcoupling interface since only the main guiding mode is excited and shallbe maintained as long as the continuity of the waveguide is ensured. Inpractice, the coupling to the high order mode from the excited mainguiding mode may occur due to the imperfectness of the waveguide, suchas the rough side wall. The excited high order modes will be removed bythe mode filter following the multimode waveguide section; and the powerloss due to this removal is equivalent to the power loss due to thescattering loss in regular single mode waveguide based PICs, since suchscattering loss is nothing but the mode coupling from the guiding modeto the radiation mode in regular single mode waveguide.

The principle of the SMC is shown in FIG. 3, which explains the reasonwhy the PIC based on the SMC architecture is equivalent to the PIC basedon purely single mode waveguide. In FIG. 3:

-   <31>: PIC based on single mode waveguide from end to end;-   <32>: PIC based SMC architecture in which multimode waveguides are    used;-   <33>: The single mode fiber at the input side of the PIC;-   <34>: The single mode fiber at the output side of the PIN;

Expression (2a) and (2b) are the insertion loss caused by the coupling,in the case of <31> and <32> respectively. In (2a), Φ_(F) is the fibermode, Ψ_(SMW) is the fundamental guiding mode of the single modewaveguide, Ψ^(k) _(R) is the radiation modes of the single modewaveguide, and η^(k) _(R) is the transmission coefficient of eachradiation mode. In principle, when the single mode waveguide length longenough, η^(k) _(R≡)0 for all k. One should notice that in (2a), theradiation modes are discrete, which is an approximate expression of thecontinuous radiation mode spectrum of the single mode waveguide. In(2b), Ψ_(MMW) _(—) ₀ is the fundamental guiding mode of the multimodewaveguide, Ψ^(k) _(MMW) and η^(k) _(MMW) are the high order mode of themultimode waveguide and its transmission coefficient. We find that oncethe radiation mode of the single mode waveguide is included, theexpression (2a) and (2b) are very similar. Mathematically, the onlydifference between single mode waveguide and multimode waveguide PIC isthat the η^(k) _(R) equals zero naturally, while η^(k) _(MMW) is notequal to zero usually, unless that we force it. By forcing η^(k)_(MMW)=0, a multimode waveguide PIC <32> can function as a single modesystem. $\begin{matrix}{\alpha_{SMW} = {\left\langle {\Phi_{F} \cdot {\overset{\_}{\Psi}}_{SMW}} \right\rangle^{2} + \left\langle {{\overset{\_}{\Phi}}_{F} + {\sum\limits_{k}\quad{\left\langle {\Phi_{F} \cdot {\overset{\_}{\Psi}}_{R}^{k}} \right\rangle{\Psi_{R}^{k} \cdot \eta_{R}^{k}}}}} \right\rangle}} & \left( {2a} \right) \\{\alpha_{SMC} = {\left\langle {\Phi_{F} \cdot {\overset{\_}{\Psi}}_{{MMW\_}0}} \right\rangle^{2} + \left\langle {{\overset{\_}{\Phi}}_{F} + {\sum\limits_{k}\quad{\left\langle {\Phi_{F} \cdot {\overset{\_}{\Psi}}_{MMW}^{k}} \right\rangle{\Psi_{MMW}^{k} \cdot \eta_{MMW}^{k}}}}} \right\rangle}} & \left( {2b} \right)\end{matrix}$

The way of forcing η^(k) _(MMW)=0 is to add a mode filter after themultimode sections (MWS). In general, a PIC using SMC structure can bedescribed as a block diagram as in FIG. 4. In the FIG. 4, the left isthe input side, and the right is output side. The first multimodesection (MWS1) is usually designed to increasing the coupling efficiency(better mode match with the input fiber or other source devices). Themode filter 1 follows the MWS1 to depress all the possible high ordermode excitements at the coupling interface. Please note this high ordermode depression will not cause any extra loss for the PIC, because allthe high order mode excitement result from the mode mismatch betweeninput fiber mode and the fundamental mode of the MWS1. This part of lossis the same as in the case of the single mode waveguide PIC, in whichthe mode mismatch part will excite the radiation modes. After modefilter 1, the MWS2 is possible for the bending purpose or modeconversion purpose. In principle, SMC architecture can have mode filterafter every MWS, but in practical, it is not necessary to do that, afterthe mode filter 1, if the following MWS sections are ideal and not highorder mode will be excited due to the imperfectness of the waveguide,the mode filter 2 or other mode filters can be skipped. In the realdesign, the excitement of the high order mode is inevitable; the modefilter must be added in the sensitive part of the SMC based PIC to avoidthe failure of the device functions. However, all these mode filterswill not affect the device performance since it simply depress the highorder mode excitement corresponding to the radiation mode excitement inthe single mode waveguide PIC.

To make the description clear, we give several SMC examples.

FIG. 5 is a waveguide lens-pinhole structure that is very similar withthat people has used in traditional free space optics. In FIG. 5:

-   <51>: Waveguide lens-pinhole PIC, in which the waveguide pinhole is    the mode filter. This PIC usually is a portion of the bigger PIC in    which it works as a input section to couple with outside fiber;-   <52>: The waveguide lens formed by etching on the edge of the PIC;-   <53>: Waveguide pinhole;

The input beam, from single mode fiber for instance, is focused by theedge waveguide lens <52>, and then propagates through a slab waveguideregion, which can be regarded as an infinite multimode channelwaveguide. A pin-hole filter <53> after the slab waveguide region willremove any multimode components excited by displacement of input beam onincident angle and the position. Although in this example, the modefilter only works for the high order mode on lateral axis, not for thehigh order mode on vertical axis.

FIG. 6 is a mode filter design example using weak-guided single modewaveguide structure, in the figure:

-   <61>: Weak Guiding Single Mode Waveguide mode filter;-   <62>: Ridge of the strong confined multimode ridge waveguide at    input side;-   <63>: Deep etched are with silicon-dioxide (or other low index    dielectric) filled;-   <64>: Shallow etched area with silicon-dioxide (or other low index    dielectric) filled;-   <65>: Weak guiding single mode waveguide section;-   <66>: Taper section for mode conversion between weak guiding section    and the multimode section;-   <67>: Ridge of the strong confined multimode ridge waveguide at    output side;

Necessary taper structure <66> is used for the transition betweenmultimode waveguide section (MWS) to the weak guiding single modewaveguide section. In the particular application like SOI waveguidebased PIC, the multimode waveguide with large cross section and deeperetch has better mode matching with the regular single mode fiber. Sothis structure can be used to reduce the coupling loss while the SMCstructure maintains the single mode operation of the whole circuits.

FIG. 7 is the example of an abnormal 3D mode size converter design,which has actually been used in the mode filter example in FIG. 6. InFIG. 7:

-   <71>: After etching, the remaining silicon surrounded by    silicon-dioxide or other low index dielectric;-   <72>: The input facet of the PIC;-   <73>: The fundamental mode profile of the waveguide at the input    facet;-   <74>: The low-level silicon slab, formed by deep etching;-   <75>: The high-level silicon slab, formed by shallow etching;-   <76>: The shoulder of the shallow etched ridge waveguide;-   <77>: The output facet;-   <78>: The fundamental mode profile of the waveguide at the output    facet;-   <79>: The abnormal mode converter;

The interface end to the fiber is a deep etch ridge waveguide that hasalmost circular mode profile, a perfect match with the guiding mode ofregular optical fiber. The deep etch ridge waveguide is multimodewaveguide in nature, then into the chip, the waveguide transfer into ashallow etched waveguide with a shoulder structure on both side.Different from the mode size converter design in prior art, in ourabnormal mode converter, the shallow ridge etch tapered in (narrower andnarrower) or not tapering at all, and the shoulder tapered out (widerand wider). The deep etch can potentially go all the way to the bottomof the original silicon slab of the SOI wafer (stop at the buried oxidelayer), in such case, the low-level slab <74> has 0 thickness.

FIG. 8 is a structure where a deep etched waveguide with strong lateralconfinement, which although it is a multimode waveguide, can be used toreduce the bending radius, therefore improve the overall circuitdensity. In FIG. 8:

-   <81>: the starting silicon slab of the SOI wafer;-   <82>: the ridge (waveguide) formed by shallow etch;-   <83>: the ridge (waveguide) formed by deep etch;

The structure in FIG. 8 can follow the structure in FIG. 6 in the realdesign of the SMC based PIC.

Parallel Mode Converter for Low Loss Optical Splitter PIC

Another structure invented here is a parallel mode conversion structureused for waveguide optical splitter, or other similar devices. As wementioned before, the SOI waveguide splitter suffers significantjunction loss due to the scattering caused by the low index materialbetween two adjacent waveguides (FIG. 2). The invented parallel modeconversion device is shown in FIG. 9. This structure can help to almosteliminate the scattering loss and split the incident power with anydesired distribution. In FIG. 9:

-   <91>: the area circled by the shallow etch outline, in which the    shallow etch will not be applied;-   <92>: the slab waveguide free space region, formed by applying    shallow etch in the area circled by deep etch outline (where deep    etch was not applied);-   <93>: the area where the deep etch was applied. This area will be    covered by the silicon-dioxide or other low index dielectrics in the    following process step;-   <94>: after the splitting, the optical mode profile of one of the    optical channels;-   <95>: the single channel mode converter (mode converter 1), which    pushes the light down to the high-level silicon slab and let the    light completely confined by the high level slab <75>;-   <96>: the waveguide for input coupling, which is multimode and    designed for coupling efficiency.

The waveguide splitter shown in FIG. 9 has two etch steps (mask layers).The first step (deep etch) creates the ridge waveguide to reduce thecoupling loss (as explained earlier in paragraph [013]). The second etchlayer is a shallow etch comparing to the first step, and it creates thesingle mode waveguide needed by the mode filter. The mode converter 1 isa taper structure in which the width of ridge created by the second step(shallow etch) gradually gets narrower until the minimum feature sizeallowed, while the width of the first-etch-created ridge is kept thesame. The mode converter will push the light down, and at the end of theconverter, the mode will be something like <102> in FIG. 10. In FIG. 10:

-   <101>: the oxide or other low index dielectric deposited on top of    silicon after all the etching steps;-   <102>: the mode profile of the waveguide at the end of mode    converter <95>;

In FIG. 10, we define two physical layers: (1) the sub layer, betweenthe surface created by the second (shallow) etch and the bottom ofsilicon slab; (2) the top layer, between the top surface of the siliconslab and the surface created by the second (shallow) etching. The sublayer is the same as the high-level slab <75> defined in FIG. 7. In thesplitter structure of FIG. 9, the light was pushed down by the modeconverter <95> and mainly confined inside the sub layer. Then itpropagates into the sub-layer free space region (a slab waveguide regionform by sub layer without lateral confinement). Inside the sub-layerfree space region, the light propagates as a circular wave (Gaussianbeam) until it reaches the parallel mode converter.

An example of the cross-section of the parallel mode converter is shownin FIG. 11, in the figure:

-   <111>: top cladding of the waveguide, for example, the TEOS (oxide)    in SOI-CMOS;-   <112>: mode profile of the supermode of the parallel coupled    waveguide at this location;-   <113>: waveguide ridge of the channels, formed by the shallow etch;

It is at the starting point that is facing the sub-layer free spaceregion <92>. Because the ridge created by the second etch is narrow, atthe starting point facing the free space, the mode is mainly confined inthe sub layer, which provides the best match with the field patterncoming from the sub-layer free space region. That is the fundamentalreason why this parallel mode converter structure can almost eliminatethe scattering loss that exists in the traditional splitter junctions.

In the parallel mode converter, after the starting point, the ridgewidth of top layer will become wider and wider. The light will thengradually move back into the top layer and be more and more confinedlaterally. At the end of the mode converter away from the sub-layer freespace region <92>, the light will be split and separated into eachindividual waveguide, as shown in FIG. 11, in the figure:

-   <121>: the waveguide ridge of one of the separated channels.

The parallel mode converter can also be regarded as a parallel reversetaper. In traditional waveguide splitters, a parallel normal taper isused to bring the light from the free space region to individualwaveguides. Parallel reverse taper achieve the same function as theparallel normal taper, but with the smallest possible scattering loss.In traditional splitters, along the aperture (the interface between freespace and the starting piont of the parallel taper), the width of thewaveguide increases from the center channel to the edge channel. The gapbetween waveguides is the minimum. At the starting point, the mode iswide and superposition of the modes of all the waveguides has theoptimized matching with the overall field pattern along the aperture ofthe free space region. Then the taper converts the wide mode at thestarting point to the strong confined guiding mode of the single modewaveguide. The reverse taper not only has wide mode at the startingpoint, but also has the mode most confined in the sub layer, therefore,no scattering loss caused by the low index material in the gap region.

FIG. 13 shows, from the starting point to the ending point, how theoptical field along the parallel mode converter is converted from aGaussian type of supermode that mainly confined in the sub-layer, to anindividual confined parallel multi-channel supermode. <131> is a 3Dillustration of parallel reverse mode converter for waveguide splitter.

FIG. 14 gives another possible variation of the parallel mode converterbased splitter structure, in the figure:

-   <141>: the starting points of the each individual channels at the    parallel mode converter;-   <142>: the lateral free propagation region, the vertical confined    provided by the sub-layer slab formed by the shallow etch;

The main difference between the structure in FIG. 14 and the structurein FIG. 9 is the starting points of the each channels at the parallelmode converter is not aligned with the wave front of the cylinder wavefrom the lateral free propagation region <142> <92>: the channels closerto the center will start later, so that, the edge channels can absorbthe light from the sub-layer earlier, to compensate the position of thechannels of being on the edge, to eventually make sure the uniform powerdistributions among the channels at the end of the mode converter.

1. Single mode photonic circuits (SMC) architecture (paragraph [007] to [014]) for photonic integrated circuits (PIC), said SMC architecture ensures the single mode operation of the whole PIC in which multimode waveguide sections are used, the said SMC architecture comprises single mode or multimode waveguide sections, optical reflection, diffraction, or guiding structures, and mode filters, the said mode filter after each multimode waveguide section or other optical guiding, diffraction, or reflection section that can cause high order mode excitation depresses the excited high order modes, and the combination of multimode section and the mode-filter section function as a single mode circuit block in the said SMC architecture.
 2. The photonic integrated circuits (PIC) using the said SMC architecture in claim 1 wherein one or more multimode waveguide or wave confined structures are used and adapted for the coupling with standard fiber or other outside optical components (FIG. 5, FIG. 6, FIG. 7) or reducing bend radius (FIG. 8), the said multimode waveguide or structure sections followed by the said mode filters force the single mode excitement to the following part of the PIC.
 3. Waveguide splitters using parallel mode conversion, or parallel reverse taper, or parallel supermode conversion (from paragraph [015] to [023]), the said waveguide splitters comprises the said multimode section for input coupling in claim 2, single channel mode converter, sub-layer slab waveguide free space region, parallel mode conversion section, and the output waveguide sections, the said single channel mode converter pushes down the lightwave to the sub-layer (FIG. 9, FIG. 10, FIG. 11), then the said sub-layer slab waveguide free space region expand the mode (optical beam), and the said parallel mode converter splits the light while bringing the light up back into the top layer (the mode is then confined in the entire silicon slab consisting of top layer and sub-layer), the said lightwave splitting process is a supermode conversion process (FIG. 13), in which the mode of very coupled parallel multi-channel waveguide is converted to the mode of very separated multi-channel waveguide, the said very coupled parallel multi-channel waveguide located at the starting point of the mode converter with narrow ridge width has the supermode matching with the expanded slab mode from the sub-layer slab waveguide free space region.
 4. Sequentially started parallel mode converter (FIG. 14) that can be used in the waveguide splitter in claim 3, the said sequentially started parallel mode converter has the starting point of each channels varying with the position of the channels in the way that the channels closing to the center starts later. 